"By obtaining baseline measurements and performing periodic evaluations, accelerated degradation can be detected before the structures and components of a storage system are unable to perform their intended function," according to the notice.

The NRC's notice pointed to two separate events involving water-damaged containers that held spent fuel from both Peach Bottom Atomic Power Station and Three Mile Island in Dauphin County.

In 2010, a low pressure alarm on one of Peach Bottom's 115-ton spent fuel casks signaled that a small amount of helium had leaked from the container.

Helium helps reduce the heat given off by the fuel assemblies and prevent corrosion.

No radiological material had leaked from the cask, and the public was not in danger from the escaped helium.

An later investigation found that water had caused the outer portion the cask's lid seal to corrode, allowing for the helium leak.

The water had flowed through an access plate on the protective cover of the cask, according to the notice.

To address the problem, the plant and the maker of the container improved the design of the plate and developed a method for verifying the integrity of the cask's protective cover.

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Water was also to blame for cracks found on the concrete storage units that protect the spent fuel and debris from TMI Unit 2's damaged core, which is stored in Idaho.

In 1999, Transnuclear installed the concrete enclosures to cover and protect dozens of canisters filled with spent fuel from TMI Unit 2, according to a 2001 NRC inspection report.

One year later, federal officials started noticing "cosmetic" or insignificant cracks in the concrete storage units, according to the inspection report.

However, in 2007, the seasonal freezing and thawing of water trapped near the roofs and in the cracks of the enclosures had started to made the problem worse.

A later independent engineering study found that, despite the cracking, the storage units remained effective in protecting the canisters, according to the inspection report.

Federal officials have repaired the cracks and are monitoring any further water damage, according to the notice.

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